Abstract

4156

Background & Purpose: St. John’s wort (SJW) (Hypericum perforatum L.) extract is a very commonly used herbal medication. Most clinical reports indicate that SJW increases cytochrome P450 3A (CYP3A) activity & decreases plasma concentrations of several drugs that are CYP3A substrates. Because docetaxel is known to be inactivated by CYP3A metabolism, we studied the effects of hyperforin, the SJW constituent responsible for inducing CYP3A & CYP2C9, on docetaxel metabolism in a human hepatocyte model. Methods: Hepatocytes were isolated from 3 donor livers by a collagenase perfusion technique. 1.5 x 106 cells in 1.5 ml Williams’ E medium supplemented with insulin, dexamethasone, penicillin/streptomycin, & amphotericin were exposed to hyperforin (0.1, 0.5, or 1.5 μM) or the known CYP inducer rifampin (10 μM) for 48 h. After 48 h, hyperforin- or rifampin-containing medium was replaced with medium containing 100 μM docetaxel. After a 1-h incubation with docetaxel, docetaxel metabolism was characterized by LC/MS/MS. Subsequent incubations of docetaxel with cloned human CYP’s characterized the specific CYP’s responsible for producing the docetaxel metabolites observed in hepatocyte incubations. Results: As expected, rifampin induced docetaxel metabolism. In the 3 hepatocyte preparations, rifampin-induced docetaxel metabolism was 6.8-, 17.5-, & 32-fold > docetaxel metabolism in corresponding DMSO-treated control cultures. Hyperforin treatment also resulted in significant increases in docetaxel metabolism in all 3 hepatocyte preparations. Hyperforin induction of docetaxel metabolism was dose-dependent &, at maximum, was 2.6- to 7-fold > that observed in DMSO-treated controls. Docetaxel metabolites identified in rifampin- & hyperforin-treated hepatocyte preparations included the previously described tert-butyl hydroxylated metabolite & 2 previously unidentified metabolites involving hydroxylation on the baccatin ring. CYP3A4 was shown to produce the tert-butyl hydroxylated metabolite as well as the 2 ring-hydroxylated metabolites. CYP2C8 was shown to produce one of the newly described ring-hydroxylated metabolites. CYP2C9 did not metabolize docetaxel. Conclusions: These results demonstrate that exposure to the SJW constituent hyperforin induces the metabolism of docetaxel in vitro. On average hyperforin appears to be 32% & 27% as potent as rifampin in inducing tet-butyl hydroxylation & ring hydroxylation, respectively. This implies that subtherapeutic docetaxel concentrations may result when docetaxel is administered to patients using SJW on a chronic basis. The results also demonstrate induction of previously undescribed metabolic pathways for docetaxel, one of which may be analogous to the known 6-α-hydroxylation of paclitaxel by CYP2C8. Support: Aventis Pharmaceuticals, American Foundation of Pharmaceutical Education, & grants P30CA 47904 & NIH NO1 DK-92310.